This invention relates to a power supply and driving circuit for a vacuum fluorescent display, and more particularly, to a power supply operable from a DC source for driving grid and plate segments, as well as heating filaments, in a large or dual vacuum fluorescent display.
Vacuum fluorescent displays are similar to vacuum tubes. A cathode, or filament/cathode combination, and a grid and plate are mounted in an evacuated glass envelope. The plate is coated with a phosphor. During operation, the heated filament emits electrons which, if accelerated by a positive potential on the grid, strike the phosphor on the plate, causing visible light photons to be emitted. The plate can be divided into a number of segments, which can be selectively charged with a positive potential to form a variety of alphanumeric characters.
Traditionally, the driving circuitry for a vacuum fluorescent display has been powered from an AC source, and has utilized either a transformer or a DC-DC/AC converter module to produce separate and stepped-up DC voltages for the grid and plate segments, as well as an AC signal for the display filament. However, the use of a transformer or converter module adds considerably to the cost and complexity of the display.
Due to the added cost and complexity associated with a transformer, display drive systems have been developed which are transformerless and operated from a D.C. source. However, these drive systems are low power/low voltage systems, such as would be utilized for a small digital clock, and are incapable of supplying sufficient power to meet the requirements of a large, dual display. Large, dual displays having in excess of 10 segments are utilized, for example, in weighing machines such as a produce scale. In order to meet the power requirements of these dual displays, a DC-DC converter must be used with the known transformerless power supplies, thereby adding significantly to the cost of the display.
Therefore, a need exists .for a power supply and drive system which can operate on a DC battery source, yet is compact, low-cost, and capable of generating sufficient power to drive a large or dual vacuum fluorescent display.